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‘Cloning’ could make structurally pure nanotubes for nanoelectronics

May be key to separating electrical conductors and semiconductors

November 16, 2012

Cloning nanotubes: In this computer model, small, pre-selected nanotube “seeds” (yellow) are grown to long nanotubes of the same chirality (twist) in a high-temperature gas of small carbon compounds (credit: Jia Liu et al./USC)

They liken this process to “cloning” the nanotubes. If it can be suitably scaled up, their approach could solve an important materials problem in nanoelectronics: producing carbon nanotubes of a specific structure to order.

Single-wall carbon nanotubes are hollow cylinders of carbon atoms bound together in a hexagonal pattern, usually about a nanometer in diameter. One fascinating feature of nanotubes is that there are many ways to wrap the hexagon sheet into a cylinder, from perfectly even rows of hexagons that wrap around in a ring, to rows that wrap in spirals at various angles — “chiralities.”

Separating electrical conductors and semiconductors

Chirality is critical to the electronic properties of carbon nanotubes. Some chiralities are electrical conductors; others are semiconductors.

“Experts in the electronics industry believe that single-wall carbon nanotubes are a promising option for nanoelectronics — semiconductor devices beyond today’s CMOS technology,” says NIST materials scientist Ming Zheng, “But for that particular application, the structure is critically important. A fundamental issue in that field is how to make single-wall nanotubes with a defined structure.”

The problem is that methods for manufacturing carbon nanotubes, which often use a metal catalyst to initiate growth, usually produce a mixture of many different chiralities or twists — along with a lot of junk that’s just soot. A lot of research has concentrated on schemes for “purifying” the batch to extract one particular kind of nanotube. And also you have to get rid of the catalyst.

Cloning nanotubes

The team led by Zheng and Professor Chongwu Zhou of USC took a different tack. NIST researchers had developed a technique for extracting nanotubes of a specific twist from a solution by using specially tailored DNA molecules that bind to one particular nanotube chirality. The DNA trick is very selective, but unfortunately only works well with fairly short pieces of nanotube.

“That approach laid the foundation for this work,” says Zheng. “We are using the short purified nanotubes to grow bigger structures of the same kind. We call it ‘cloning,’ like cloning an organism from its DNA and a single cell, but in this case, we use a purified nanotube as a seed.”

Small segments of nanotubes of identical chirality, extracted using the DNA technique, were put in a high-temperature reaction chamber at USC with methane gas, which breaks down in the heat to smaller carbon molecules that attach themselves to the ends of the nanotubes, gradually building them up — and preserving their structural chirality. “A bit like building a skyscraper,” Zheng observes, though in these early experiments, the tubes are laying on a substrate.

“I think the most important thing this work shows is that from a chemistry point of view, it’s entirely possible to grow nanotubes without a catalyst, and even maintain control of the structure,” says Zheng, “It’s a different approach, to do the separation first to obtain the ‘seeds’ and then do the synthesis to grow the desired nanotubes.”

The research was funded in part by the Semiconductor Research Corporation’s Focus Center Research Program, Functional Engineered Nano Architectonics, and the Office of Naval Research.

Comments (6)

This is only slightly related to this article’s topic, but if you’re interested in IBM’s research into using nanotechnology to develop neuromorphic processors, you should take a look at the papers Dharmendra Modha posted a few days ago on his website.

They’re using petascale supercomputers to simulate lots and lots of their cognitive computing chips, totaling 65 billion simple neurons for one experiment, and then 530 billion in another. He’s going to post the papers detailing machine learning experiments they did with the chip simulator.

It is my impression that, mathematically speaking, carbon nanotubes and even theoretical materials made from them are not “strong enough” in principle for use as space elevator cables. I would hope to be mistaken in this perception.

“A bit like building a skyscraper,” Zheng observes…or maybe a bit like growing a cable. When cables of any length and diameter can be cloned, then an asteroid, a carbonaceous chondrite, can be parked in geosynchronous orbit and the cable for a space elevator — a “skyhook” — can be lowered to the Earth.